Buy FFXIV when it's on sale for cheap and open a second account? If we assume that additional slots cost the same as a retainer ($2 per slot), it's actually be cheaper to just get another account in the long run, especially if another 8 characters are made.

As a refresher, the standard subscription costs for non-legacy people are:
30 days: 14.99
60 days: 13.99
180 days: 12.99

So another 8 characters with it's own account would still be cheaper than adding 8 slots at $2 each for 30 days.

The same caveats would still apply. You'd need need to transfer them to Balmung, which bunches of new players already are doing. It'd at least offer you the chance to temporarily unsubscribe from an account of all the characters you were currently playing were on one account as well. (Assuming veterans awards are not a concern).